With the continued concern regarding the quality of the environment, industry, government, and academia have joined forces to find methods of treating various forms of contamination for the purpose of minimizing their impact on the environment. Methods that have been described in the past include bioremediation, such as bacteria that consume hydrocarbons, however these processes tend to be expensive, very selective, complex, slow and typically can't handle the more difficult "refractory" materials such as polyhalophenols. Physicochemical processes such as extraction, adsorption, stripping, evaporation, and flocculation and precipitation have been employed for the purpose of decontamination, but in some cases the contaminated media must be removed from the site and further treatment required. These processes also tend to be energy inefficient. Lastly, chemical treatment of contaminated media, such as the use of chlorine, chlorine dioxide, ozone, and "per" compounds such as potassium permanganate, have been employed to treat contaminated media. However, these treatments utilize hazardous materials, do not necessarily generate innocuous intermediate or end products, and are typically expensive.
One form of chemical treatment that has shown promise as a viable decontaminating system is Fenton's Reagent. Fenton's Reagent is comprised of hydrogen peroxide and ferrous salts, the latter react with hydrogen peroxide to generate hydroxide radicals. These radicals are powerful oxidizing agents and will react with a plethora of materials that are amenable to oxidation decomposition. In the case of organic materials the process will generate carbon dioxide and water. Examples of contaminants include, organic dyes, nitrocresols, chlorinated phenols, formaldehyde, hydrocarbons such as gasoline fuel, AOX aromatic compounds such as BTEX (benzene, ethylbenzene, toluene, and xylene), chlorinated hydrocarbons including PCBs and dioxins, organic acids, organometallic compounds containing metals such as lead, mercury, copper, chromium and others, insecticides, fungicides, microorganisms including E. coli and various viruses, bacteria and pathogens, miscellaneous waste streams, and groundwater containing high levels of carbon oxygen demand (COD) and biological oxygen demand (BOD-5) materials. Both COD and BOD-5 materials are not necessarily toxic, per se, but their oxygen consumption in lakes and streams lowers the natural oxygen level to the point where flora, fauna, and aquatic life cannot survive.
Although Fenton's Reagent can be used to treat a wide range of contaminants, it suffers from several well known drawbacks including formation of iron sludges which either must be further treated or handled in a prudent manner to prevent leaching/dissolution. Furthermore, hydrogen peroxide is expensive and is inherently unstable when maintained at elevated temperatures for long periods. Lastly, the decontamination process using Fenton's Reagent is slow, sometimes requiring elevated temperatures and resident times from one-half to five hours as reported by Yamanaka et al, U.S Pat. No. 4,624,792; Toshikuni et al, U.S. Pat. No. 4,693,833; and Carr et al, U.S. Pat. No. 4,604,234. Various techniques have been employed to speed the decontamination process when using Fenton's Reagent including supplying electrical energy or electromagnetic radiation. The problem of using electromagnetic radiation (typically in the ultraviolet region) is it cannot be used as a bulk treatment since radiation absorption occurs especially at the surface. Therefore, large amounts of surface area are required and even in these cases the process is inefficient and costly.
U.S. Pat. No. 4,131,526 to Moeglich et al discloses use of an electrolytic cell to decontaminate oxidizable contaminants. Oxidation takes place using alternating current that is varied within the range of 0.5 to 800 Hertz. Below 0.5 Hertz, Moeglich reports that fouling of the electrodes occurs, limiting the life of the process. The electrodes used by Moeglich include stainless steel, graphite, titanium coated with ruthenium dioxide or manganese dioxide, amorphous carbon and platinum. Moeglich prefers using a central electrode made of graphite surrounded by a stainless steel mesh electrode. As further required by the process, oxidizing catalysts are added to the electrolytic cell as powders of metal oxides of Group IVa, Va, VIb, and VIIb including, germanium, tin, lead, antimony, bismuth, chromium, molybdenum, tungsten, manganese, and rhenium. These powders are incorporated onto carrier particles which are then added to the cell. Due to this design, it is preferred that no large solid particles be present because they could interfere with the cell operation by causing plugging. In the practice of U.S. Pat. No. 4,131,526, the amount of insolubles, other than in a colloidal phase, must be held to less than 1% and most preferably less than 100 ppm. The invention also relates to the production of hydrogen peroxide which may be consumed in the oxidation of oxidizable compounds in the aqueous medium. This invention suffers from the required use of expensive metal oxides preferably on carrier particles. In one embodiment, these particles are in a fixed bed limiting contact with the bulk of the contaminated waste stream or effluent, thereby reducing efficiency.
Wabner, in U.S. Pat. No. 4,834,852 describes another electrolytic cell having a direct current density of 0.5 to 50 mA/cm2, where hydrogen peroxide in the range of 0.5 to 10 mg/L per COD unit is employed as the oxidant for conversion of degradable or toxic substances. The cathode of the dielectric cell is made of special steel, titanium, nickel or graphite. The anodes can be graphite or dimensionally stable valve metal anodes (DSA). It is a feature of the invention that the electrodes have a sufficiently long useful lifetime (i.e. do not dissolve under processing conditions). No metal or metallic salts are added as a catalyst, so this process, although utilizing lower levels of hydrogen peroxide, is inherently slow.
Fischer, et al, in U.S. Pat. No. 5,068,038 addresses the problem of ferric hydroxide sludge formed during the decontamination process by using silicic acid having a specific surface area above 50 m2/gm as a second adsorption means. Approximately 20-1000 mg/L of ferrous salts are required to be added during the oxidation process. Although increased efficiency of decomposition of AOX is obtained, the system still generates a ferric hydroxide sludge which requires further decontamination before discarding.
Vignieri in U.S. Pat. No 5,520,483, teaches the in situ remediation of groundwater contamination using Fenton's Reagent. The process as described requires the lowering of the pH of the contaminated media with acetic acid at a flow rate of 5-10 gallons/minute (GPM). The total amount of acetic acid is typically 1-3% by volume of the effective volume of the contaminated water. Ferrous ion solution is also added at 0.5-3%, by volume, of the effective volume of the contaminated water. Lastly, hydrogen peroxide is added at 1-5%, by volume, of the effective volume of the contaminated water. Potable water must also be injected into the wells between addition of the three components to allow for a sufficient plume to develop for the process to work effectively. A plurality of wells must also be used to ensure significant decomposition. No use of electrolytic amplification was contemplated by Vignieri. The process as described does not actively disperse the hydrogen peroxide and consequently the hydrogen peroxide can remain close to the injection wells and can result in explosive concentrations. Second, diffusive methods require a long time for the reagents to diffuse and therefore the remediation process is slow to complete.
Wilson, in U.S. Pat. No. 5,525,008 obviates the impediments in Vignieri, U.S. Pat. No. 5,520,483 by gas pressure injecting through a single or plurality of injectors the oxidizing agents into the soil as well as the groundwater. Under these conditions diffusion is rapid and increased by the evolution of gases such as CO2, air or oxygen. Successful remediation of the affected area was reported to be obtained over the course of a week. Furthermore, no electrolytic augmentation of the oxidative decontamination was contemplated by Wilson, et al.
Gnann, et al, in U.S. Pat. No. 5,538,636 addresses the issue of ferric hydroxide sludge, formed as a by-product during the oxidative decontamination of highly contaminated wastewaters when using Fenton's Reagent. A process is disclosed that eliminates the sludge by electrolytically treating the sludge to reduce the ferric salts back to ferrous salts. This allows for recycling the ferrous salts back into the oxidative process thereby minimizing sludge waste. The oxidative process is performed using anodes that are dimensionally stable (e.g. titanium, platinum, and metal oxides), while cathodes are composed of steel mesh or carbon. Preferably the molar ratios of COD/H202/Fe are in the range of 20/20/1 to 20/10/5. The regeneration process for converting ferric hydroxide to ferrous salts requires several steps to be performed including precipitation of the ferric salts with soda lime. This is followed by separation of the treated wastewater using a side channel pump and feeding it through a PE crossflow filter. The time required for these processes can be as long as 6 hours and requires careful balancing of the steps in order to have a viable process.
Jasim, et al, in U.S. Pat. No. 5,716,528 describes the treatment process for wood preservative effluents comprising chlorinated phenols and polynuclear aromatic hydrocarbons (PAH). Jasin et al teaches that hydrogen peroxide, and ferrous salts, in acidic media, can be used to decompose wood preservative effluents. The weight ratio of hydrogen peroxide to COD is within the range of 0.9:1 to 1.25:1 and the concentration of ferrous ion is suitably about 5% based on hydrogen peroxide weight. The best results for the decontamination process are at elevated temperatures, approximately 40 C but at this temperature decomposition of hydrogen peroxide occurs. Precipitation of ferric hydroxide is necessitated because high levels of hydrogen peroxide and thereby ferrous salts are used to treat the concentrated effluent. For greatest degree of destruction of pentachlorophenol, a ratio of peroxide to pentachlorophenol of 27:1 is required, however for unsubstituted phenol, the ratio is 3:1. Reaction times are reported between 3 to 3.5 hours and no electrolytic augmentation was contemplated.
The examples hereinabove describe the current technology for hydrogen peroxide oxidative decomposition of various aqueous waste streams. A common problem throughout the prior art is that when ferrous salts are used in appreciable quantities, the additional treatment step to remove ferric salts is required to make the process environmentally acceptable. Furthermore, augmentation of Fenton's Reagent in oxidative decontamination processes by the use of electrolytic energy can enhance the rate of the decontamination process but problems exist with fouling of the electrodes and the requirement that the electrodes be made of materials that are essentially inert to the process thereby requiring fairly expensive materials to be used. Other important attributes for a process of detoxification or decontamination should include:
a) ability to treat various types of contaminants, PA1 b) rapid processing of less than one minute PA1 c) cost efficient, PA1 d) energy efficient, PA1 e) high throughput, PA1 f) safe to operate, and PA1 g) the process itself should generate minimal wastestreams requiring limited further treatment PA1 h) no toxic intermediates PA1 i) elimination of iron salts, and PA1 j) no need to adjust pH.